This document discusses phase changes and latent heat. It explains that during phase changes, heat is absorbed or released but temperature remains constant. The heat energy during phase changes is used to change the bonding between molecules rather than their kinetic energy. As a result, temperature stays the same even as heat is added or removed. The document provides examples of different phase changes like melting, freezing, evaporation and condensation. It also defines latent heat as the "hidden" heat absorbed or released during phase changes without changing temperature. Specific values of latent heat are given for various substances.

1. CHANGE OF PHASE
AND LATENT HEAT
Prepared by: Merie Lyn DJ. Agustin MAE - Physics

2. CHANGE OF PHASE
 The term “change of phase” means the same
thing as the term “change of state”.
There are four states, or phases, of matter:
•Solid
•Liquid
•Gas
•Plasma
THERMODYNAMICS

3. CHANGE OF PHASE
 When a substance changes from one state or phase of matter to
another we say that it has undergone a change of state, or we say
that it has undergone a change of phase. For example, ice melts and
becomes water; water evaporates and becomes water vapor.
These changes of phase always occur with a change of heat. Heat,
which is energy, either comes into the material during a change of phase
or heat comes out of the material during this change. However, although
the heat content of the material changes, the temperature does not.
THERMODYNAMICS

4. Why Do Phase Changes Occur?
Phase changes typically occur when the temperature or
pressure of a system is altered.
Example:
 At normal atmospheric pressure, ice melts as temperature
increases.
 If you held the temperature steady, but lowered the
pressure, eventually you would reach a point where the ice
would undergo sublimation directly to water vapor.
When you supply heat to a substance or you remove heat from it,
then you can cause the substance to change its state.

5. Here are the five changes of phase:
Description of
Phase Change
Term for Phase
Change
Heat Movement
During Phase
Change
Temperature
Change During
Phase Change
Solid to Liquid Melting Heat goes into the solid
as it melts.
None
Liquid to Solid Freezing Heat leaves the liquid as
it freezes.
None
Liquid to Gas Evaporation Heat goes into the liquid
as it vaporizes.
None
Gas to Liquid Condensation Heat leaves the gas as it
condenses.
None
Solid to Gas Sublimation Heat goes into the solid
as it sublimates
None

6. So, how could there be
a change in heat
during a state change
without a change in
temperature?

7. During a change in state, the heat energy is used
to change the bonding between the molecules. In
the case of melting, added energy is used to
break the bonds between the molecules. In the
case of freezing, energy is subtracted as the
molecules bond to one another. These energy
exchanges are NOT changes in kinetic energy.
They are changes in bonding energy between the
molecules.

8. If heat is coming into a substance during a phase
change, then this energy is used to break the
bonds between the molecules of the substance.
The example we will use here is ice melting into
water. Immediately after the molecular bonds in
the ice are broken the molecules are moving
(vibrating) at the same average speed as before,
so their average kinetic energy remains the same,
and, thus, their Kelvin temperature remains the
same.

9. Breaking the bond of the molecules as heat is added to the material.

10. PHASE CHANGE DIAGRAM
Freezes
Melts
Heat Energy
Condenses
Boils
Cools
In a similar way heat enters a liquid
to change the molecular bonding
when the liquid boils or evaporates
into a gas, and heat enters a solid to
change the molecular bonding when
it sublimates into a gas.

11. In the ice, the molecules are strongly bonded to one another, thus forming a rigid
solid.
When heat is added to the ice these bonds are broken and the ice melts. The molecules
afterward bond to one another with less strength and a different geometry, and water is
formed.
Now, before the melting, the molecules were actually moving when in the solid state.
They were vibrating back and forth. They had an average kinetic energy. So they had a
Kelvin temperature proportional to this average kinetic energy.
PHASE CHANGE DIAGRAM
Heat Energy
Melts
After the melting, the water molecules are still
vibrating. And they have the same average
kinetic energy as they had before the melting.
So, the water is at the same temperature at the
moment after the melting that the ice was at the
moment before the melting.

12. In none of these changes of state is the
heat (energy) that is input or output
used to change the speed of the
molecules. The average speed of the
molecules is the same before and after a
phase change, and so is the average
kinetic energy.

13. Heat
(energy)
is
transferre
d into the
ice.
The heat is
used to break
the bonds
between
molecules, not
to increase
the average
kinetic energy
of the
molecules.
Since the bonds
among the ice
molecules have
been broken, water
is formed. The water
molecules, at this
moment, have the
same average
kinetic energy as
they did when they
were ice.
Since the ice
and water
molecules
both have the
same average
kinetic energy,
they are at the
same Kelvin
temperature.
So, how could there be a change in heat during a
state change without a change in temperature?

14. All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.

15. There are three primary phases of matter: solid, liquid
and gas. A solid becoming liquid is called melting or
fusion. A solid becoming gaseous is called
sublimation. A liquid becoming solid is called freezing.
A liquid changing to gas is called boiling or
evaporation. A gas changing into a solid is called
deposition, and a gas changing into a liquid is called
condensation. Half of these are endothermic, meaning
they absorb heat from their surroundings. The others
are exothermic, meaning they release heat.

16. All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.
PHASE
CHANGE
NAME INTERMOLECULAR
FORCES
Increase or Decrease?
SOLID
LIQUID
Melting
or
Fusion
 Increase
or
 Decrease

17. All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.
PHASE
CHANGE
NAME INTERMOLECULAR
FORCES
Increase or Decrease?
LIQUID
GAS
Evaporation
 Increase
or
 Decrease

18. All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.
PHASE
CHANGE
NAME INTERMOLECULAR
FORCES
Increase or Decrease?
GAS
SOLID
Deposition
 Increase
or
 Decrease

19. All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.
PHASE
CHANGE
NAME INTERMOLECULAR
FORCES
Increase or Decrease?
GAS
LIQUID
Condensati
on
 Increase
or
 Decrease

20. All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.
PHASE
CHANGE
NAME INTERMOLECULAR
FORCES
Increase or Decrease?
SOLID
GAS
Sublimation
 Increase
or
 Decrease

21. Very Good!!!
For any given substance,
intermolecular forces
will be greatest in the solid state and
weakest in the gas state.
All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.

22. All phase changes involve either an increase or decrease of intermolecular forces. For
each of the phase changes below, choose whether intermolecular forces increase or
decrease.
PHASE
CHANGE
NAME INTERMOLECULAR
FORCES
Increase or Decrease?
LIQUID
SOLID
Freezing
or
Crystallization
 Increase
or
 Decrease

23. LATENT HEAT
It is the “hidden” heat when a substance absorbs or releases heat without
producing a change in temperature of the substance, e.g. during a change of
phase.
When a substance changes its state from a solid to liquid or from a liquid
to a gas heat energy is needed. This energy is used not to heat up the
substance but to separate the molecules from each other.
This energy is called LATENT HEAT energy.
While a solid is melting and while a liquid is boiling there is no
temperature change. The temperature only changes when the change of
state is complete.

24. Substance
(at 1 atm)
Melting
Point
Tm (K)
Latent Heats Table
Heat of
Fusion
Lf (cal/g)
Boiling
Point
Tb (K)
Heat to
Vaporize
Lv (cal/g)
Hydrogen 13.8 14.0 20.3 108
Oxygen 54.4 3.3 90.2 50.9
Nitrogen 63.3 6.1 77.3 48.0
Ethyl
156 24.9 351 205
Alcohol
Mercury 234 2.7 630 70.0
Water 273.15 79.7 373.15 539
Lead 600 5.9 2023 208
Aluminum 932 94.5 2740 2500
Gold 1336 15.4 2933 377
Copper 1359 32.0 1460 1210
Iron 1808 69.1 3023 1520

25. Specific Heat Latent Heat of Fusion
Material (cal/g °C) (J/kg K) (cal/g) (J/kg)
Aluminum 0.215 900 94.5 3.96x105
Copper 0.092 385 49.0 2.05x105
Iron 0.107 448 63.7 2.67x105
Lead 0.031 130 5.5 0.23x105
Brass 0.092 385 Unknown Unknown
Magnesiu
m 0.245 1030 88.0 3.7x105
Zinc 0.093 390 27.0 1.1x105
Styrofoam 0.27 1131 Unknown Unknown
Air 0.240 1006 N/A N/A
Water 1.000 4190 N/A N/A
Ice 0.500 2095 79.7 3.34x105
Specific and Latent Heat Values

26. Latent Heat of Fusion or Liquefaction
The amount of heat required per unit mass of a
substance at melting point (constant) and under 1
Atmosphere pressure to convert it from the solid to
the liquid state is called Latent Heat of Fusion.
L = Q/m Calorie/gm or K.Calorie/kg

27. When a substance is changing its state, the temperature of the substance remains
constant. For example boiling water at 100 °C on changing state becomes steam
(vapour) at 100 °C
Latent heat
The heat energy that is taken in or
given out by a substance when it
changes state is called latent heat.
When a substance changes from solid
to liquid, the latent heat involved is
called the latent heat of fusion. When
the substance changes from a liquid to
a vapour, latent heat of
vaporisation is involved.

28. LATENT HEAT
1. How much heat energy is needed to
change 2.0 kg of ice at 0°C to water at 0°C?
QL = mL
QL = ( 2.0 kg ) ( 3.3 x 105 J/kg)
QL = 6.6 x 105 J
EXAMPLE PROBLEMS

29. LATENT HEAT
2. How much heat energy is needed to
change 0.50 kg of water at 100°C to steam
at 100°C?
QL = mL
QL = ( 0.50 kg ) ( 2.3 x 106 J/kg)
QL = 1.2 x 106 J
EXAMPLE PROBLEMS

30. 3. How much heat does a refrigerator need to remove
from 1.5 kg of water at 20.0 °C to make ice at 0°C?
[Hint: find heat removed for water at 20.0°C to water at 0°C, then find latent heat for
water at 0°C to ice at 0°C, and add the t]
Q total = mcΔ T + ml
EXAMPLE PROBLEMS

31. Heat with Temperature Change
Q = mcΔT
Q = (1.5 kg)(4.2 x 103 J/kgºC)(20.0ºC)
Q = 1.25 x 105 J
Latent Heat
QL = mL
QL = (1.5 kg)(3.3 x 105 J/kg)
QL = 4.95 x 105 J
The total amount of heat needed is:
Q + QL
1.25 x 105 J + 4.95 x 105 J
6.2 x 105 J
EXAMPLE PROBLEMS

32. Latent Heat Applications
Water has one of the highest latent heat of fusion values of all substances and
therefore has several applications.
1. Picnic Coolers
The specific latent heat of fusion for water is 3.3 x 105 J/kg. That is, 3.3 x 105 J of
heat are needed or absorbed to melt 1 kg of ice. The heat required to melt the ice
comes from the food or drinks in the cooler. Since heat leaves the food, it gets cold.
2. Preventing Frost Damage
When 1 kg of water freezes it releases 3.3 x 105 J of heat energy. Farmers use this
principle to prevent frost damage to their orchards and other crops. When a frost is
predicted, farmers will turn on the water sprinklers. As the water falls on the plants
and starts to freeze, heat is released to the surroundings and plants. The heat helps
the plants stay warm enough to prevent damage. This only works when the
temperature does not drop much below freezing.

33. 3. Cooling Off When Wet
You can cool off your body by just coming out of a shower or swimming pool
if you do not dry off with a towel. It takes 2.3 x 106 J of heat to vaporize 1 kg
of water. When you are wet, heat from your body is used to vaporize the
water. Because heat leaves your body, you feel cool. If you don't want to
cool off, just dry yourself well with a towel.
4. Avoid Steam Burns
When steam condenses it releases heat to the surroundings. When just 1
mL of steam water condenses it releases 2.3 x 103J of heat. Then the
condensed water cools from 100°C to 37°C (body temperature), so even
more heat is released and absorbed by the skin. That's why a steam burn is
much worse than a burn from boiling water.

34. Heat absorbed or released as the result of a phase change is called latent heat. There is no
temperature change during a phase change, thus there is no change in the kinetic energy of
the particles in the material. The energy released comes from the potential energy stored in
the bonds between the particles.
Q = mL
EXOTHERMIC (warming processes)
condensation
warmer in the shower
steam radiators
freezing
orange growers use ice to stop oranges from freezing
deposition
snowy days are warmer than clear days in the winter
ENDOTHERMIC (cooling processes)
evaporation/boiling
sweat
alcohol is "cool"
melting
melting ice in drinks
sublimation
cooling with dry ice

35. Summary
All phase changes …
take place at a specific temperature.
take place without a change in temperature. (There is no
temperature change during a phase change.)
involve changes in internal potential energy.
release or absorb latent heat.
Endothermic phase changes absorb heat from the
environment. (They are cooling processes.)
Exothermic phase changes release heat to the environment.
(They are warming processes.)

36. Scattered thoughts …
Under extreme conditions of heat and exercise, an individual may sweat more than a liter of
liquid per hour.
The interior of roasted meat can never reach temperatures higher than the boiling point of
water until all the water is cooked out of it, at which point it would resemble shoe leather. The
outside is quickly dried out, however, and can reach the temperature of the surrounding
cooking medium.
Cocoa butter is unique among the fats in that it is very regular in composition; whereas most
other fats are actually mixtures. This gives it a very definite point; unlike butter, which softens
gradually. As it melts in your mouth, it absorbs latent heat. This makes chocolate bars taste
"cool". Cocoa butter is remarkably uniform in composition and structure: only three fatty acids in
the majority of its triglycerides, with the same one occupying the middle position. Pure cocoa
butter is quite brittle up to about 34 (93 ), at which point ℃ ℉ it melts quite quickly.

37. The specific latent heat (L) of a material …
is a measure of the heat energy (Q) per mass (m) released
or absorbed during a phase change.
is defined through the formula Q = mL.
is often just called the "latent heat" of the material.
uses the SI unit joule per kilogram [J/kg].
There are three basic types of latent heat each associated with
a different pair of phases.

39. So, let’s look at the whole process together in the form of a graph. The graph
describes how the temperature of 1 gram of water in its three forms (ice, water,
steam) changes as heat is added or taken away.
A to B: pure ice
B to C: a mixture of water and ice all at 0 ºC
C to D: pure water
D to E: a mixture of water and steam all at 100 ºC
E to F: pure steam

40. EXAMPLE PROBLEMS

41. EXAMPLE PROBLEMS